Morphology of the Blastoidea. 281 



by Wachsmuth and Springer *, all of them skilled palgeon- 

 tologists and of acknowledged ability as accurate observers. 

 Mr. Hambach's reasons for denying the existence of the 

 summit-plates described by these authors were partly theo- 

 retical and based upon the supposed resemblance of Pen- 

 Semites to "Echinoderms" by which he apparently meant 

 the Echinoidea. He has replied to my criticism of his views, 

 which he has elaborated somewhat more fully than before. I 

 propose to discuss them further on, and will only remark at 

 present that theoretical morphology is not a very safe guide 

 as regards questions of fact f. 



Mr. Hambach's second argument against the existence of 

 these summit-plates was founded upon the fact that he 

 possesses specimens in which the supposed plates " prove to 

 be Bryozoa or ovulum-like bodies " ±. He now explains § 

 that the last expression had reference to the oolitic character 

 of the rock in which the Pentremites were imbedded, and he 

 totally denies the accuracy of Shumard's figure of a Pentre- 

 mites conoideu-s || in which the central opening is closed by a 

 group of six plates, a central one and five others disposed 

 interradially around it. Is he prepared in like manner to 

 question the accuracy of Homer's description and figure of 

 the summit of Elceacrinus Verneuili% , in which there is a 

 central hexagonal plate surrounded by six others, two of 

 which are in the anal interradius and one in each of the 

 remaining interradii *? He admits that " the condition of life 

 was undoubtedly a similar one throughout the whole class " of 

 the Blastoidea** ; and as he totally denies the existence of 

 summit-plates in Pentremites, he is bound to give some 

 explanation of those described by Homer in Elceacrinus. 

 Are they ovulum-like bodies, Bryozoa, or what ? I have 

 myself examined a good many specimens of Elceacrinus, and 

 am convinced that Komer's description is correct and that the 



See also " Remarks on the Blastoidea, with Descriptions of new Species," 

 Proc. Acad. Nat. Sci. Philad. 1869, pp. 84, 85. 



* " Revision of the Palseocrinoidea, Part II.," Proc. Acad. Nat. Sci. 

 Philad. 1881, p. 207, pi. xix. tig. 3. 



t A good illustration of this is the nervous nature of the axial cords 

 of a Crinoid, which is now generally admitted, although it has been 

 strenuously denied on theoretical grounds, because there was no place 

 for it in the established archetype of an Echinoderm. 



% Trans. St. Louis Acad. vol. iv. p. 150. 



§ Ibid. p. 541. 



|| Ibid. vol. i. pi. ix. fig. 4. 



f Op. cit. p. 58, Taf. v. figs. 1 b, 1 e. 



** Trans. St. Louis Acad. vol. iv. p. 547. 



